While I'm riding, my mirrors tend to shift to the point that I can't really see anything behind me, and I have to adjust them again so I can see. This wouldn't be a major problem, but I'm having to do this every 2 or 3 minutes, which is especially difficult while on the highway. I've tried tightening the allen bolts where they swivel, but they won't tighten any more. I read elsewhere on the forums that you can add a bit of thread lock to keep them in, but I was wondering if anyone has actually tried this and had it work? Are there any other solutions to this?

When I first got my bike it did that too, but only when I was going 90-100mph. I just tightened the Allen bolts and that worked fine for me, maybe check to see if the screws are in there straight?

I had some aftermarket mirrors (doesn't everyone with a 20 year old bike?) and they had this problem too. The ball joint where the mirror meets the standoff got too lose and the aero pressure would push the mirror to give me a reflection of myself (unflattering).

I squeezed the cup (attached to the standoff) with vice grips and that tightened it right up. It's also free and kinda ghetto, which is EXACTLY how I roll.

I've had some mirrors do that as well. There's no real way to fix them. You could try some thread lock. I'd just figure out where you want them and put in one of the stronger ones so they don't move, ever.

Otherwise I'd just pick up a new pair of mirrors. I do remember riding and all of a sudden one of my mirrors would just swing loose.

Get some thread lock into the pivots. After a couple days it gets gooey and thick. Really helps. It worked for my F series mirrors. I had the same problem with the Denali fog lights moving. Again thread lock to the rescue.
Remember the difference: thread lock gets gooey and allows you to remove things later. Lock-Tite stuff gets hard like epoxy. You don't want that (well, thread lock is also made by the Lock-Tite company and others, so read the package before buying.)

Thanks for the replies! Just checking though, if I put thread lock into the base, the allen bolt can still be removed later, right? Just don't want to do anything that's irreparable 

Right. Thread lock is like gooey sludge when it sets up in a day or two. It doesn't get rock hard like "Lock-Tite". Remember to read the package as Lock-Tite corp. also makes thread lock. The package will say thread lock. There are different grades or amount of gooeyness of threadlock. You can still remove the bolts.
Really, you should use thread lock on just about every screw, nut and bold on a bike so they don't vibrate loose.
If there is a lot of space in the mirror mount, slip in a thin washer. Those mounts are so heavy duty that I didn't find it practical to try to squeeze them tighter together. A thin washer if needed and always thread lock did the trick.

Perfect, thanks!